Plaster of Paris casts have been used to immobilize body members for some time. These bandages are made by depositing plaster of Paris on a reinforcing scrim material such as gauze. When the plaster of Paris is dipped in water, the alphahemihydrate of calcium sulfate is converted to calcium sulfate dihydrate, which results in the hardening of the cast. Plaster of Paris casts, however, suffer from a number of disadvantages. X-ray transmission through the cast to determine whether a fracture has been properly set is extremely difficult. In addition, the cast is quite heavy and restricts the mobility of the patient wearing the cast. The casts are also very sensitive to water and may seriously lose their load-bearing capability if they become wet. In addition, the air permeability of the plaster of Paris cast is very limited, and, as a result, they do not allow evaporation of moisture from the skin beneath the cast, which may result in skin irritation beneath the cast.
In order to overcome the disadvantages of plaster of Paris casts, numerous attempts have been made to develop plastic or plastic-reinforced materials as a replacement for plaster of Paris.
U.S. Pat. Nos. 3,241,501 and 3,881,473 disclose casts which are made with a flexible fabric impregnated with a polymer which is capable of being cured by ultaviolet light. Although this casting material overcomes some of the disadvantages of plaster of Paris cast material, it requires a different technique in its application and also requires the use of an ultraviolet light source in order to cure the cast. These casts also require significantly longer times for the cast to set before they will be load bearing.
More recent attempts to produce substitutes for plaster of Paris include the casting tapes made with polyurethane prepolymers which are disclosed in U.S. Pat. Nos. 4,376,438; 4,427,001; and 4,411,262. These casting tapes are made from relatively open weave fabrics coated with polyurethane prepolymers, that is, reaction products of isocyanates and polyols. The bandages are dipped into water in the same manner as the plaster of Paris bandages and then applied to the limb of a patient. The water causes the prepolymer to polymerize and form a rigid polymer structure which will support the broken limb. In order to obtain the desired rapid hardening or setting of the bandage, there are usually catalyst systems employed in the prepolymer formulation. The above-mentioned U.S. Pat. No. 4,376,438 employs an amino polyol which in effect is a catalyst built into the polyol portion of the polyurethane. The above-mentioned U.S. Pat. Nos. 4,411,262 and 4,427,002 employ added catalysts such as amines to catalyze the hardening of the polyurethane prepolymer. U.S. Pat. Nos. 4,433,680 discloses a similar diisocyanate polyol composition which employs a dimorpholinediethylether as the catalyst.
The polyurethane prepolymer casting tapes described above have met with considerable acceptance as a replacement for plaster of Paris. The general properties of the casting tapes and the finished casts have been found to be acceptable to both physicians and patients and offer advantages over plaster of Paris cast bandages.
One of the problems connected with the manufacture of such polyurethane casting tapes is the balance between the gel time of the casting tapes on storage and the set time of the casting tapes after they are activated with water and applied to the patient. If the catalyst concentrations are excessive or if certain types of catalysts are used to cause the casting tape to set quickly, there is a danger that the polyurethane will gel or precure while it is still in the package which results in an unacceptable shelf life for the product. The mere reduction of the catalyst levels of certain types of catalysts will overcome this problem, but may result in very long set times for the finished cast on the patient. The set time is generally the time which it takes for the cast to harden and it must be relatively hard before any weight can be put on the cast by the patient. p Another problem with such casting tapes is that the set times are generally too long to be employed in certain situations. With the casting of the broken limbs on infants the set time must be quite short in order to ensure that the casting tape will harden before the limb of the patient is moved.